1. Field of the Invention
The present invention relates to techniques for the generation of negative ion beams, and in particular, to the generation of such ion beams by attachment of material to be detected to zero and near-zero velocity electrons. Such techniques are particularly useful for ultra-sensitive detection of vapors by molecular electron capture as used, for example, in the detection of concealed explosives or narcotic compounds.
2. Description of the Prior Art
Certain heavy molecules may be detected at very low vapor concentrations by the detection of negative ions produced by attachment with ultra low or zero energy electrons. As shown in U.S. Pat. No. 4,649,278 co-invented by the applicant hereof, near zero energy electrons produced by applying a beam of electrons from an electron gun to a mirror electrostatic field attach to molecules in a gas sample to form negative ions which may then be detected in accordance with their mass signatures by mass analysis.
An improved version is shown in U.S. Pat. No. 4,933,551, also co-invented by the applicant hereof, in which a relatively complex, high-current, in-line reversal electron ionizer capable of focusing a beam of electrons to a reversal region is used to produce zero and near-zero energy electrons for the production of detectable negative ions by attachment to certain heavy molecules in low vapor concentration.
The use of the Reversal Electron Attachment Detection--or READ--technique for detecting vapors from explosives and other compounds is well described in the literature, for example, in "Reversal Ion Source: A New Source of Negative Ion Beams", Orient, Chutjian and Alajajian, Rev. Sci. Instrum. 56 (1) Jan 85, pp 69-72; "Pulsed, High-Current, In-Line Reversal Electron Attachment Detector", Bernius and Chutjian, J. Appl. Phvs. 66 (7) Oct. 1989, pp 2783-2787; and "Application of Reversal Electron Attachment for Ultra-Sensitive Detection of Thermal Electron-Attaching Molecules: CC1.sub.4 and C.sub.6 H.sub.5 N0.sub.2 ", Bernius and Chutjian, Analytical Chemistry, 62 (13) July 1990, pp 1345-1349.
As the techniques for detecting concealed narcotics or explosives compounds improve, however, so do the techniques for concealing such compounds. What are needed therefore are less complicated, more sensitive techniques which may readily be produced, deployed, maintained and serviced in many locations, such as airports, harbors, embassies, etc.